1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, and more particularly to control of glossiness of an image produced by the image forming apparatus.
2. Description of the Background Art
As is well known, an image forming apparatus using electrophotography forms an electrostatic latent image on a photoreceptor serving as a latent image bearing member and develops the latent image with toner into a visible image, also known as a toner image. The toner image is then transferred onto a recording medium such as a sheet of paper and fixed thereon.
Such an output image includes not only a monochrome image, but also a color image consisting of multiple colors. The appearance of the image and reproducibility of the colors depend significantly on glossiness of the image as well as glossiness of the recording medium. It is known that when glossiness of the image corresponds to glossiness of the recording medium, the output image appears to be natural. That is, one may feel that the image has a sense of uniformity. Thus, it is necessary to match the glossiness of the recording medium and the image.
Attributes that determine glossiness include factors such as fusing ability and permeability of the toner relative to the recording medium. In order to fuse the toner to the recording medium, a fixing device is used.
Generally, there are two fixation methods employed by the fixing device for fixing the toner image: a fixing method using a heating roller and a fixing method using a belt. The fixing method using the heating roller uses a fixing roller, equipped with a heat source such as a halogen lamp or the like inside the fixing roller, and a pressure roller that contacts the fixing roller. The fixing method using the belt uses a fixing belt having a relatively small heat capacity.
In the fixing method using the belt, the fixing belt includes an elastic layer formed of silicone rubber or the like on which a separation layer of fluorocarbon polymer is formed. The fixing belt is wound around a plurality of rollers and stretched therebetween. A pair of rollers consisting of a stretch roller and a pressure roller is disposed such that the stretch roller faces the pressure roller through the fixing belt, thereby defining a fixing nip. The heat source such as a halogen lamp is provided inside the stretch roller.
In such a fixing device, the recording medium bearing the toner image thereon is transported between the fixing belt and the pressure roller. As the recording medium passes therebetween, the toner in different colors is heated and fused, thereby fixing the color toner image on the recording medium.
The fixing method using the belt is advantageous in that the belt member provides greater flexibility in forming the fixing nip, enabling fixation of the toner image at low temperature (thus saving energy) and enhancing separability of the recording medium from the fixing belt.
Additionally, there are various types of recording media sheets on which such a toner image is fixed. For example, the recording media include, but are not limited to, ordinary paper, art paper, coated paper, semi-coated paper, and so forth. Ordinary paper or the like has a relatively rough surface. In other words, surface asperity is significant so that glossiness of the ordinary paper is low. However, in recent years, market demand has also grown for an ability to produce high-quality images on art paper, coated paper, and slightly-coated paper, the surfaces of which are relatively smooth, that is, surface asperity is small and glossiness is high. Generally, the surface of art paper and coated paper has a coating layer formed of resin or the like. Therefore, glossiness is high compared with ordinary paper. Also, in recent years, in order to add a quality appearance to a paper document, use of matte-coated paper having glossiness similar to that of ordinary paper is increasing.
Therefore, there is demand for an image forming apparatus that can reliably form a quality image on various types of paper. As noted above, conventionally glossiness is considered to be one of several attributes that determine the quality of the overall appearance and color reproducibility of an image. In order to achieve desirable glossiness, a balance between glossiness of the recording medium on which no image is formed (hereinafter referred to as “non-image area”) and the gloss of an image area of the recording medium where the image is formed (hereinafter also referred to as “high density portion” or “toner image portion”) needs to be optimized.
Conventionally, glossiness of an image is evaluated or quantified using a 60-degree glossiness scale according to Japanese Industrial Standards (JIS). In this method, glossiness is measured at a 60-degree angle from the horizon. Using this method, when the measured glossiness of the recording medium and the measured glossiness of the image portion correspond to each other, it is believed that an image with a desirable gloss is obtained.
However, the present inventor has noticed that the perceived gloss may not coincide with the measured glossiness using the conventional method, and glossiness of various kinds of sheets of recording media and the image portion of such media cannot be defined by a generally known single indicator/numerical value “glossiness”. One reason for such divergence between the perceived gloss and the measured glossiness is that the difference in refractive indices of the recording medium and the toner of the image portion causes a significant difference in the total amount of light reflected from the surface of the recording medium.
Moreover, even if the measured glossiness of the recording medium and the image portion correspond numerically, one may still feel that gloss between the recording medium and the image portion lacks uniformity.
In order to obtain a desirable gloss, in one related-art approach, a toner fixing speed is adjusted to control fusing of the toner image based on the surface roughness of the recording medium. In this approach, the distribution of the light reflected by the recording medium (reflected light distribution curve) is obtained to specify the surface roughness Ra of the recording medium. Accordingly, fixing conditions can be adjusted to fix the toner image onto the recording medium using only an appropriate amount of heat.
Although advantageous, this approach suffers from a drawback. For example, even if the distribution of the light reflected by the recording medium is detected, the distribution of the light reflected by the image portion is not measured. Consequently, it is not possible to set the distribution of the reflected light to a similar if not the same distribution for both the recording medium and the image portion.
In another related-art approach, in order to achieve a desirable gloss of the toner image and the recording medium, an image is formed to satisfy a standardized predetermined distribution of specular reflection light, and the fusing ability of the toner is then adjusted to achieve that standardized predetermined distribution of the specular reflection light.
Although this approach focuses on the distribution of the reflected light (an angle that is the half value of a reflected light peak), the standard of reference is glass plate. In other words, this approach focuses on achieving the high gloss image (texture) of silver halide photographs rather than coated paper. Furthermore, this approach determines mainly whether or not the produced image satisfies standardized predetermined conditions. As such this approach does not propose how to obtain appropriate gloss on different kinds of recording media sheets.
Alternatively, there is another approach in which an image is formed by incorporating information on image glossiness and texture using not only the specular reflection light but also upper diffuse reflection light.
However, this approach relates to a reading device and does not address a problem associated with the use of “glossiness” as a parameter for evaluation of gloss when the reflected light flux differs between different kinds of recording media sheets. Also, this approach does not suggest obtaining the distribution of the light reflected from the non-image area to correspond to the image area (toner image area).
Conventionally, with high-gloss paper, a user may need to try out different kinds of output modes such as a “gloss paper mode”, “a thick paper mode”, and so forth to make gloss of the non-image portion and the image portion have a sense of uniformity. Yet even despite the effort, a desirable gloss may still not be obtainable.
As described above, it is difficult to determine whether or not the recording medium that the user wishes to use can provide desirable gloss by using the conventional “glossiness” as an indicator.